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Experimental subjects (n = 29) were patients who had undergone orthodontic treatment in combination with extraction of maxillary or mandibular premolar teeth, or both. Control subjects (n = 29) were healthy dental students with no orthodontic or extraction experience. Sagittal (corrected axis) tomograms of the TMJs were used to determine the narrowest linear distances between the anterior and posterior outlines of the TMJ condyle and the TMJ fossa, expressed as the joint space ratio. There were no significant (p greater than 0.05) differences between the control and experimental ratios. Bipolar surface electromyograms of the masseter and anterior temporalis muscles were used to determine the isometric contraction velocities of these muscles until 50% and 100% voluntary isometric contraction effort (teeth clenching) was achieved. There were no significant (p greater than 0.05) differences between the control and experimental subjects. Electromyograms were also used to determine the relative contribution of the masseter and anterior temporalis muscles to the bite force developed during brief maximum voluntary tooth clenching, expressed as the activity index. There were no significant (p greater than 0.05) differences between the control and experimental subjects.

To study the possibility of interactions between buccal cutaneous sensory receptors and voluntary maximum isometric contractions of the masseter muscles, six adult subjects exercised maximum teeth clenching before and after spraying the right cheek surface with aerosol containing 20% benzocaine. The right cheek and masseter muscle served as the experimental side, the left cheek and masseter muscle as the control side. Isometric motor outputs, on the right and left sides, were monitored by integrated surface electromyography over periods of 10 seconds. Topical surface anesthesia provided no evidence of motor modulation by cutaneous tactile receptors. Before and after anesthesia, the two muscles showed nearly identical and well-coordinated motor innervation patterns. It is suggested that the cortical motor commands of maximum isometric contractions, with recruitment of practically all available motor units, overrule all modulatory inputs except those of fatigue.

A randomized, single-blind crossover trial tested the effect of 1,000 mg of ibuprofen on jaw muscle pain induced by maximum voluntary teeth clenching (MVC). Subjects exercised MVC until there was onset of pain in the masseter muscles (pain latency in seconds), and until pain and exhaustion of the masseter and anterior temporalis muscles could no longer be endured (pain tolerance in seconds). Pain intensity was quantified by visual analogue scores, and pain sensitivity by the pain sensitivity range and the pain sensitivity ratio. During MVC the mean voltage of the left masseter muscle was recorded by cumulative surface electromyography. Ibuprofen had no significant effect on the pain latency and the pain tolerance. Neither did ibuprofen significantly decrease the pain intensity nor significantly affect the pain sensitivity range and the pain sensitivity ratio. After intake of ibuprofen, the number of electromyograms with a decrease in mean voltage was significantly increased--credibly, an expression of increased central fatigue with voluntary decruitment of motor units, and possibly the result of increased contraction times because of an undisclosed effect of ibuprofen. There was no circumstantial evidence of impaired motor activity that could be attributed to biosynthesis of prostaglandins.

To study the reliability of nonfatiguing maximum voluntary static work efforts by the masseter muscle, six healthy subjects exercised teeth clenching in centric occlusion. Maximum voluntary teeth clenching was performed for 10 seconds on 2 different days, each with two trials, and maximum static work efforts (without artificial feedback-control) were quantified by integrated surface electromyography. Reliability was determined by factorial analyses of variance and intraclass correlations. Data reduction showed that maximum voluntary static work efforts were reproduced reliably during the four different trials.